Today, there are many systems that incorporate so called GPS-receivers, which utilize the satellite navigation system, GPS (Global Positioning System), in order to receive accurate information about, for example, the speed, direction or position of the GPS-receiver, and thereby also the device, system, vehicle etc. into which the receiver is potentially incorporated, such as for example an airplane, a car, a boat or even inside such small devices as cellular mobile telephones etc.
However, a well-known problem with GPS-receivers is that, while it is simple to determine, for example, the position in an outdoor environment with a high degree of accuracy using a GPS-receiver, it is often nearly impossible to determine the position in an indoor environment by using the GPS-receiver. This is because the received signal-strength of the GPS-signal is seriously attenuated by the walls and the roof of a building wherein the GPS-receiver is located.